Don’t Part-Out That Cuda! Save It With A New Crossmember!

It’s probably not something you’ve ever really thought about unless you’ve seen your Mopar stripped down to a bare metal shell and you realize that your entire car is made out of nothing but formed sheet metal held together by spot welds.

It’s actually a rather interesting piece of work by Chrysler engineers that reduces weight and yet increases strength by turning pretty much every part of the body into a structural component instead of using a single frame as a foundation for the rest of the car.

What about the frame rails, you ask? Aren’t they at least a partial frame under the car? Sorry, they too are just formed sheet metal held together with spot welds like the rest of the body. Granted, they are made from 14-gauge material instead of the 20-gauge used for the rest of the body panels but they are still formed sheet metal just the same.

Of course these “heavier” parts do bear a greater portion of the load so it’s vital that they are not rust-compromised in any way. One of these that normally doesn’t get a lot of attention is the torsion bar crossmember. Tying the mid-point of the car together at the rocker panels, it also supports the entire torsional sprung weight of the front of car via the back ends of the torsion bars that mount within it.

While they don’t tend to rust as quickly as their body panel counterparts, they are not immune to it and Muscle Car Restorations of Chippewa Falls, WI had determined that the cross member on this ’71 ’Cuda was going to have to come out. These are not the kind of parts to even think about patching in a way that would be fine for other body panels. Fortunately, there is no need to as AMD can provide these “frame” parts just like the rest of the body panels you might need.

Like the rest of the body, the frame rails and crossmember don’t rust in isolation. The floor in this car was also beyond any type of patch repair. This actually works in your favor as removing the floor greatly facilitates swapping out the crossmember.

The car must be solidly supported, preferably on a frame rack, with whatever additional bracing is necessary to insure that nothing can move out of place when major components like these are removed. Since it is under the car and out of sight, it could be assumed that the precise positioning necessary for the rest of the visible body panels wouldn’t be needed with this. In part that’s true. Cosmetically, it doesn’t matter, but functionally it has to be correct so that the torsion bars fit without binding in any way.

It’s actually a very real benefit that the floor of this ’71 Plymouth ’Cuda is beyond repair. Being able to simply cut it all out of the way makes replacing the crossmember a fairly straight forward job.One of the keys to MCR’s success with this kind of work is building custom jigs for each job that provide positive locator points for the new parts. These jig posts are built to fit in existing holes in the crossmember.Like so many other body parts, rust damage is often hidden from normal view inside the part, as you can see here where the front frame rail meets the crossmember.It’s important to note that the crossmember (and the frame rail) is not complete in this form. It’s only when the floor is welded on top of it that it reaches its full designed strength.Since no part of it is being saved, there is no reason to take the crossmember out in one piece, so MCR simply cuts it out with a Miller plasma cutter.Not unsurprisingly, the front frame rail also needs replacing so the end of it was cut off to release it from the crossmember. The spot welds at the end of the crossmember-to-rocker-panel joint could be easily accessed now but since the rockers will also be replaced, that won’t be necessary.Here you can see the importance of the jig posts that MCR attaches to the frame rack. They positively locate both the crossmember and the frame rail. The gold paint is Copperweld Weld-Though primer. MCR applies this to all surfaces that will be welded together to prevent rust from forming in between the seams.This is a very important step: The lower control arms are mounted on the K-frame and both torsion bars slide through the crossmember and fit in the control arms to check for proper alignment. You are also confirming that the crossmember wasn’t installed backwards or binds the torsion bars in any way which would obviously cause enormous frustration later.With the floor installed, the crossmember and surrounding boxed-in area is back to full strength. MCR’s ProSpot spot welder can reach all the way to the trans tunnel but plug welding the floor to the crossmember is perfectly acceptable.From the bottom, it’s easy to see how the crossmember, frame rail, and rocker panel all work together to form a unibody version of a frame. This arrangement is actually stronger than a body-on-frame design.This is why it’s called a torsion bar crossmember: The rear end of the torsion bar fits into this hexagonal socket. All of the spring tension from the working of the front suspension is transferred to this spot, so you can see that it’s imperative that this part of the body be correctly aligned and at its full designed strength.